Combustion and free-radical substitution

Reaction types and bond fission

Organic reactions are classified as addition, substitution, elimination, oxidation, reduction and polymerisation.

• Homolytic fission: a bond breaks with one electron going to each atom, forming two free radicals (species with an unpaired electron, shown with a dot).
• Heterolytic fission: both electrons go to one atom, forming a positive and a negative ion.

An electrophile is an electron-pair acceptor.

Combustion

Complete combustion gives CO2 and H2O: C3H8 + 5O2 -> 3CO2 + 4H2O. With limited oxygen, incomplete combustion gives CO or carbon (soot).

Free-radical substitution

Methane and chlorine react in UV light.

• Initiation: Cl2 -> 2Cl• (UV breaks the Cl-Cl bond homolytically)
• Propagation: Cl• + CH4 -> HCl + •CH3, then •CH3 + Cl2 -> CH3Cl + Cl•
• Termination: two radicals join, for example Cl• + •CH3 -> CH3Cl, or •CH3 + •CH3 -> C2H6

Further substitution gives CH2Cl2, CHCl3 and CCl4, and termination gives longer chains, so the method gives a mixture and has limited use for making one product.

Hazard and risk

A hazard is something that could cause harm (a flammable or toxic substance). Risk is the chance of harm actually happening. Risk is reduced by working on a smaller scale, using a fume cupboard, avoiding naked flames, wearing eye protection, and using less hazardous substances where possible. A risk assessment should be done before any practical.

Worked example

Write the two propagation steps for the reaction of ethane with bromine in UV light.

1. A bromine radical removes an H atom.
2. The ethyl radical reacts with a Br2 molecule, regenerating Br•.

Answer: Br• + C2H6 -> HBr + •C2H5; •C2H5 + Br2 -> C2H5Br + Br•

Check yourself

1. Which equation shows initiation in the chlorination of methane?
   • Cl• + CH4 -> HCl + •CH3
   • •CH3 + Cl• -> CH3Cl
   • Cl2 -> 2Cl•
   • CH4 -> •CH3 + H•
   Show answer

   Cl2 -> 2Cl•. UV light breaks Cl-Cl, the weakest bond, into two radicals.

2. Which step is a termination step?
   • •CH3 + •CH3 -> C2H6
   • Cl2 -> 2Cl•
   • •CH3 + Cl2 -> CH3Cl + Cl•
   • Cl• + CH4 -> HCl + •CH3
   Show answer

   •CH3 + •CH3 -> C2H6. Two radicals combine and no radical is left.

3. What happens in homolytic fission?
   • Each atom keeps one electron from the bond, forming two radicals
   • One atom takes both electrons, forming ions
   • A new bond forms
   • An electron pair is donated
   Show answer

   Each atom keeps one electron from the bond, forming two radicals. Homo means the same: the bond splits evenly.

4. Why is free-radical substitution of limited use for making chloromethane?
   • It needs a nickel catalyst
   • Further substitution and termination steps give a mixture of products
   • It has 100% atom economy
   • It only works in the dark
   Show answer

   Further substitution and termination steps give a mixture of products. CH3Cl can react further to CH2Cl2 and beyond, and radicals combine in many ways.

5. What is the difference between a hazard and a risk?
   • They mean the same thing
   • Risk is a substance, hazard is a procedure
   • A hazard is the chance of harm; risk is the substance
   • A hazard could cause harm; risk is the chance of that harm happening
   Show answer

   A hazard could cause harm; risk is the chance of that harm happening. Precautions reduce risk without changing the hazard.

6. How many moles of O2 are needed to burn 1 mol of butane, C4H10, completely?
   Show answer

   6.5 mol. C4H10 + 6.5O2 -> 4CO2 + 5H2O: O atoms needed = 8 + 5 = 13, so 6.5 mol O2.

7. Which is a propagation step?
   • Cl• + Cl• -> Cl2
   • Cl2 -> 2Cl•
   • •CH3 + •CH2Cl -> CH3CH2Cl
   • Cl• + CH3Cl -> HCl + •CH2Cl
   Show answer

   Cl• + CH3Cl -> HCl + •CH2Cl. A radical reacts with a molecule and a new radical forms.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.